This invention relates generally to monolithic fuel cells and is particularly directed to the fabrication of multilayered, thin-walled cellular ceramic structures for use in high temperature monolithic solid oxide fuel cells (SOFC).
In monolithic fuel cells, fuel and oxidants are combined electrochemically in a ceramic cell at an operating temperature of 1100.degree.-1300.degree. K. Cell components are fabricated as one piece, much like a block of corrugated paper board. Fuel and oxidants are conducted through alternating passages in the stack, with the passages formed from thin (0.025-0.100 mm) layers of the active cell components: electrolyte (yttria-stabilized zirconia), cathode (strontium-doped lanthanum manganite), anode (nickel-zirconia cermet), and the interconnection material (magnesium-doped lanthanum chromite) that connects cells in electrical series (bipolar plate). The corrugations also form the gas seal at the edges of the structure. Advantage is taken of the ability to fabricate the solid electrolyte and other solid cell components into shapes that cannot be achieved in liquid electrolyte systems. In liquid electrolyte systems, much of the mass and volume goes into building the inert container for the liquid. Eliminating this unnecessary material gives the monolithic fuel cell a significant advantage in performance.
The potential for high power density of the monolithic fuel cell results from the small cell size. Cells 1 to 2 mm in diameter or smaller are achievable when the inert container for electrolyte and inert support for the thin active layers are eliminated. The small cell size increases the active surface area per unit volume of the cell and reduces the voltage losses due to internal electrical resistance. Voltage loss reduction is a significant consideration, because internal resistance is the principle dissipative loss for the ceramic materials and temperatures of interest. Decreasing the cell size decreases the current path length, because current is carried in-plane by the electrodes in the monolithic design.
One solid oxide fuel cell design which shows promise for commercial application is illustrated in FIG. 1. This approach makes use of an SOFC module 10 comprised of alternating layers of anode 16/electrolyte 12/cathode 14 composite which is corrugated, and cathode 14/interconnection 18/anode 16 composite which is flat. Fuel and oxidant flow in alternate sections of the corrugations on opposite sides of the anode/electrolyte/cathode composite. This design provides a high power density because it has high electrolyte surface area per unit volume. However, the manifold arrangement for this design is relatively complex. Another SOFC design illustrated in FIG. 2 includes alternative flat layers of anode/electrolyte/cathode composites and cathode/interconnection/anode composites separated by corrugated anode and cathode layers to provide the fuel and oxidant flow passages. The anode and cathode corrugations are oriented at 90.degree. relative to one another to simplify fuel and oxidant manifold design. The strong, lightweight honeycomb structure of the small cells of this approach are currently being evaluated for space applications and show commercial promise. The small size of these cells and their high output power density makes this approach particularly attractive.
Because this SOFC structure includes various components comprised of different materials in a relatively complex multilayered structure, attempts to fabricate these SOFCs in large scale, commercial type production runs have met with only limited success. Many of the problems encountered are associated with thermal stress and bonding involving thin structures containing different materials with varying physical properties. These problems are primarily exhibited in the form of cracks in the electrolyte and interconnection layers which allow fuel and air to mix and reduce cell performance. The extent of these problems can be reduced by individual firing of the various layers of the SOFC, but this multi-step approach is time consuming, increases the cost of the fuel cells, and is thus of limited commercial utility.
The present invention addresses the aforementioned problems of the prior art by providing an SOFC arrangement and method of fabrication which makes use of intermediate layers having a unique composition between the differing composition of the electrodes and electrolyte which substantially reduces thermal stress and bonding problems during SOFC fabrication. This approach is particularly adapted for single firing at a given temperature of the entire SOFC structure during cell fabrication. The cell structure is self-supporting while in the green state which facilitates the firing process.